The operators of communication networks, such as the conventional telephone network, provide the subscribers or their customers not only with the transmission of useful data (in the telephone network: the voice) but often also with a multiplicity of further complex services or subscriber service features. These services include conference calls, the transmission or suppression of telephone numbers, call forwarding or the metering pulse, for example.
To provide these extensive services, the subscriber line module (SLM) needs to comprise various functions. One of these functions is what is known as the “reversal” function or inversion function, that is to say the possibility of reversing the polarity on the usually two wires (a and b) of a subscriber line. This function is used for transmitting a metering pulse, for example.
A communication network's subscriber line module is usually arranged at the exchange end. In addition, it usually comprises two units per connected subscriber, the subscriber line interface circuit (SLIC) and the encoder/decoder, known as the CODEC.
In this case, the subscriber line interface circuit is the direct exchange-end access point for the subscriber line. The two wires of the subscriber line are physically connected to the subscriber line interface circuit and are supplied with the signals required for transmitting the useful data by it. The subscriber line is usually a conventional, commonly known twisted copper pair.
The CODEC converts the analog signals from the subscriber end into digital signals which are sent from the exchange and onward via the communication network. Similarly, digital signals coming from the exchange are converted into analog signals and are forwarded to the subscriber line interface circuit.
The CODEC usually accommodates additional further elements. Thus, the CODEC contains what is known as a reversal generator, for example. This is responsible for implementing the reversal function, that is to say for reversing or inverting the polarity at the subscriber end.
When inspecting the “reversal” function of the subscriber line module, it is necessary to check, inter alia, whether the subscriber line module actually provides inverted polarities on the wires of the subscriber line when the reversal function has been activated.
In conventional methods which can be attributed to the prior art, this check is performed using an external measuring instrument. In this case, the reversal function is activated and the measuring instrument connected to the subscriber line is used to check whether the polarity on the wires actually changes.
One drawback of such a solution, inter alia, is the need for the external measuring equipment described above, however, which needs to be operated appropriately by additional personnel.